Optical fibers have acquired an increasingly important role in the field of communications, frequently replacing existing copper wires. This trend has had a significant impact in the local area networks (i.e., for fiber-to-home uses), which has seen a vast increase in the usage of optical fibers. Further increases in the use of optical fibers in local loop telephone and cable TV service are expected, as local fiber networks are established to deliver ever greater volumes of information in the form of data, audio, and video signals to residential and commercial users. In addition, use of optical fibers in home and commercial business for internal data, voice, and video communications has begun and is expected to increase.
The fibers used in local networks are directly exposed to harsh conditions, including severe temperature and humidity extremes. Prior coatings for optical fibers did not perform well under such adverse conditions. The need existed for the development of higher performance coatings to address the wide and varied temperature and humidity conditions in which fibers are employed. Specifically, these coatings possessed thermal, oxidative, and hydrolytic stability which is sufficient to protect the encapsulated fiber over a long life-span (i.e., about twenty-five or more years).
Optical fibers typically contain a glass core, a cladding, and at least two coatings, i.e., a primary (or inner) coating and a secondary (or outer) coating. The primary coating is applied directly to the cladding and, when cured, forms a soft, elastic, and compliant material that encapsulates the glass fiber. The primary coating serves as a buffer to cushion and protect the glass fiber core when the fiber is bent, cabled, or spooled. Stresses placed upon the optical fiber during handling may induce microbending of the fibers, which can cause attenuation of the light that is intended to pass through them, resulting in inefficient signal transmission. The secondary coating is applied over the primary coating and functions as a tough, protective outer layer that prevents damage to the glass fiber during processing and use.
Certain characteristics are desirable for the primary coating, and others for the secondary coating. The modulus of the primary coating must be sufficiently low to cushion and protect the fiber by readily relieving stresses on the fiber. This cushioning effect must be maintained throughout the fiber's lifetime.
Because of differential thermal expansion properties between the primary and secondary coatings, the primary coating must also have a glass transition temperature (Tg) that is lower than the foreseeable lowest use temperature. This enables the primary coating to remain elastic throughout the temperature range of use, facilitating differences in the coefficient of thermal expansion between the glass fiber and the secondary coating.
It is also important for the primary coating to have a refractive index that is different (i.e., higher) than the refractive index of the cladding. This permits a refractive index differential between the cladding and the primary coating that allows errant light signals to be refracted away from the glass core.
The cost to produce coated optical fibers with the above properties is largely dependent on the draw tower line speed and draw utilization. A limiting factor in the operation of a draw tower line speed is the rate of cure of the primary and secondary coatings applied to the fibers. Under cured coatings can yield unwanted fiber defects, which may lead to functional problems with the resultant coated fiber. Previous methods to improve the rate of cure or rate of polymerization include the use of wholly acrylated coating systems, the use of highly efficient photoinitiating systems, and increases in UV radiation. Photoinitiated polymerization reactions generally follow the relationship:Rp=kp[M](ΦεIo[A]b/kt)1/2 where Rp: rate of polymerization; kp: propagation rate constant; [M]: concentration of reactive functional group; Φ: quantum yield for initiation; ε: molar absorptivity; Io: incident light intensity; [A]: concentration of photoinitiator; b: thickness of reaction system (coating thickness); and kt: termination rate constant.
Formulation efforts to maximize the rate of polymerization include the use of reactive monomers, oligomers, and mixtures thereof with high propagation rate constants, the use photoinitiators with high photoinitiating efficiencies, and selecting components that would not increase the tendency toward chain termination or chain transfer. Chain transfer agents may not decrease the rate of polymerization, but will reduce the degree of polymerization.
Two prior approaches for improving the rate of cure involve the use of aliphatic glycidyl(meth)acrylate monomers or aromatic glycidyl(meth)acrylate monomers. Unfortunately, aliphatic glycidyl(meth)acrylate monomers are characterized by low refractive indices, resulting in primary coatings with refractive indices that are lower than desired. Aromatic glycidyl(meth)acrylate monomers, such as phenoxy glycidyl acrylate, have the disadvantage of imparting a high Tg to cured compositions comprising these monomers. This renders the cured compositions unsuitable for use as a primary coating composition in most applications.
Thus, a need still exists to identify other monomers that can improve the rate of cure, while achieving a primary coating composition that possesses a desirable refractive index and a desirable Tg.
The present invention is directed to overcoming these and other deficiencies in the art.